1. Field of the Invention
The present invention relates to integrated circuits and more specifically to input/output contact pads formed on the last metallization level of integrated circuits.
2. Discussion of the Related Art
FIGS. 1 and 2, respectively, are a partial top view and a corresponding cross-section view of the last metallization level of an exemplary integrated circuit. A coil 1 and a contact pad 2 are formed on a substrate 3. Substrate 3 is an insulating layer covering a previous metallization level or a semiconductor substrate. Coil 1 is spiral-shaped in top view and five portions 5, 6, 7, 8 and 9 are visible in FIG. 2. Contact pad 2 is square-shaped in top view as current in integrated circuits. A passivation layer 10 covers the coil and substrate 3, as well as the edges of contact pad 2. An opening 11 in passivation layer 10 exposes a central portion of contact pad 2.
The forming of the last metallization level of an integrated circuit such as shown in FIGS. 1 and 2 consists of covering substrate 3 with a metal layer, generally made of aluminum, then etching this metal layer to form coil 1 and contact pad 2, and finally covering the entire structure with a passivation layer in which an opening is formed above contact pad 2.
The development of integrated circuit manufacturing processes enables placing more and more elements on the same substrate surface area. On a given metallization level, the width of the conductive strips is smaller and smaller. Similarly, the size and the spacing of the contact pads are decreased to increase their number.
FIG. 3 is a cross-section view of the contact pad of FIG. 2 after welding of a metal wire. The tip of the metal wire substantially has the shape of a ball 20 resting on contact pad 2. Upon welding, ball 20 is laid on contact pad 2, and a strong force, as well as ultrasound, are applied to form an intermetallic welding area 21 between ball 20 and contact pad 2.
Since aluminum is a relatively flexible material, the application of a strong force and of ultrasound causes a penetration of ball 20 into contact pad 2 and results on the one hand in the forming of cracks 22 in passivation layer 10, and on the other hand in the forming of aluminum projections 23 on either side of ball 20, some of the projections rising above passivation layer 10. The presence of cracks 22 and of aluminum projections 23 is likely to cause short-circuits between the input/output contact pads generally placed next to one another and this, becomes more likely as the pads get closer and closer. This results in reliability problems for the concerned integrated circuits.